The thin film transistor with a layer of metal oxide semiconductor as the active channel layer (MOTFT) has attracted great attention for its high carrier mobility and for its potential for next generation displays and thin-film electronics. However, contemporary issues remaining to be solved include operation stability of the current-voltage characteristics of such transistors in dark and under light illumination. These issues are more profound in devices with related high mobility. Due to the difference between broad-band ionic semiconductors and narrow-band covalent semiconductors, the underlying mechanisms of the instabilities in MOTFTs are fundamentally different from those observed in a-Si TFTs.
For a metal oxide TFT under negative bias temperature stress, the metal oxide can go through reduction (i.e. losing oxygen) with the presence of electrons and water leading to a negative shift in threshold voltage (Vth).e−+MO+H2O→M++2OH−M+ loss of oxygen Vth→negativeWater decomposition in the presence of strong negative gate bias to the metal oxide channel layer, in turn, provides additional electrons due to the following reaction:H2O→2H++O+e−
As the channel is depleted of electrons by the negative gate bias, the above process is accelerated from left to right. Therefore, the channel becomes more conductive and a large negative shift in Vth is observed under negative bias temperature stress (NBTS).
This deleterious effect of water or moisture on negative gate bias stress stability is particularly profound when the TFT is under illumination in which many electrons and holes are generated. One of the strategies to reduce negative bias temperature stress is to limit the presence of water, which at the present time is accomplished chiefly by having a good passivation around the TFT. However, it is difficult and costly to provide a perfect barrier (passivation) to water. Furthermore, any short wavelength light that can be absorbed by the metal oxide semiconductor channel layer has to be blocked in order to reduce the optically induced electrons in the channel layer. In active matrix display applications, it is also difficult to perfectly block light from reaching the channel layer. Some small amount of light will get into the metal oxide layer through scattering and wave guiding. While it may be argued that the amount of moisture entering the TFT and the amount of light impinging on the TFT are small, it must be understood that these effects are occurring over the entire life of the TFT. Therefore, an additional method is desired to reduce the sensitivity of Vth shift under negative bias temperature stress with stray light impinging on the metal oxide.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved MOTFT with reduced sensitivity to Vth shift under negative bias temperature stress (NBTS) with impinging stray light, and at the same time with improved stability under positive bias temperature stress (PBTS) and higher mobility.
It is another object of the present invention to use the new and improved MOTFT for thin film electronic circuits and for electronic devices/apparatus comprising such thin film circuits.
It is another object of the present invention to provide new and improved methods and apparatus for reducing the sensitivity of Vth shift under either positive or negative bias temperature stress with or without stray light impinging on a MOTFT.